Pressure control valves are used, for example, as ABS control valves in an antilock braking system in order to enable a pressure difference to be set at the wheel brakes of a vehicle, in particular a commercial vehicle, as soon as a case of brake slip is detected. In this case, the pressure control valve or the ABS control valve comprises an inlet valve and an outlet valve, each of which is designed as a 2/2 directional solenoid valve. Depending on the control action, which takes place in a clocked or pulsed manner, the inlet valve or the outlet valve is in either an open or a closed state, and so, when controlled accordingly, a brake pressure can be increased, maintained, or reduced at the particular wheel brakes of a vehicle wheel.
An ABS control valve of this type can also be used, however, for regulating the brake pressure when a demanded brake pressure or a brake pressure difference is specified by any type of driver assistance system or by a stability control system, for example, a stability program (ESP), an adaptive cruise control (ACC), a rollover stability control (RSC), an emergency braking system (AEBS), an anti-slip regulation (ASR) or a yaw control (YC). Additionally or alternatively, in the event of driver braking, the brake pressure or a brake pressure difference can be specified at individual wheel brakes by a brake pressure distribution (EBD) or a brake force limitation (EBL) and can be controlled by the ABS control valve.
The control of the pressure control valve or the ABS control valve takes place in this case via a control device which controls, i.e., opens or closes, the inlet valve and/or the outlet valve for a certain pulse duration. In this case, the pulse duration determines the pressure difference which is set at the wheel brakes by the control, via the pulse duration. Depending on whether the outlet valve and/or the inlet valve are/is controlled, the brake pressure at the wheel brakes is therefore reduced or increased by the amount of the pressure difference, or the pressure is maintained, depending on the pulse duration. The correlation between the pulse duration and the pressure difference is based, both for the inlet valve and for the outlet valve in this case, on a previously empirically determined pressure difference characteristic curve which is determined in advance, in each case, for the corresponding ABS control valve.
The disadvantage thereof is that the particular pressure difference characteristic curve for the inlet valve or for the outlet valve is recorded at a certain temperature of 20° C., for example. The pressure difference characteristic curve is also temperature-dependent, however, due to the temperature dependence of sealing elements, lubricants, and other components of the ABS control valve, and so pressure differences controlled by the ABS control valve differ, for a fixed pulsed duration, at an ambient temperature of 20° C. and, for example, −20° C. As a result, a reliable braking according to the predefined pressure difference cannot be ensured at an ambient temperature that deviates from 20° C. In addition, the control action of a driver assistance system or a stability control system is therefore very sluggish and unreliable at very low temperatures, for example, of less than 0° C.
In order to take the temperature dependence into account in the control of the pressure control valve, it is provided in DE 37 29 183 C2 to measure a coil current at a measurement resistor, which coil current is flowing through the solenoid valve of the magnetically actuated pressure control valve. This coil current is a measure of the operating temperature of the valve, in particular of the temperature of the flow medium. A temperature is assigned to the measured coil current and, on the basis of the temperature, an adjusted pulse duration is determined, with which the pressure control valve is controlled. The disadvantage thereof is that the adjustment of the pulse duration carried out solely as a function of the temperature of the solenoid coil is not accurate, since the temperature of the solenoid coil resting on the outside of the pressure control valve does not correspond to the temperature of the valve body and, therefore, can vary according to external influences.
A further temperature-dependent adjustment is provided in WO97/29001. According thereto, the pulse duration is corrected depending on variables which influence dynamics of the pressure change, wherein the variables are, in particular, an ambient temperature, a hydraulic temperature, a supply voltage at the pump motor, or the pressure generated by the pump motor. In this case, the dynamics of the pressure change, i.e., the pressure build-up or the pressure decrease, are considered depending on the temperature which, according to the prior art, changes only as a result of the temperature of the pressure medium.